1. Field of the Invention
The present invention relates to dissolved oxygen sensors, and more particularly to a dissolved oxygen concentration monitoring system which provides automatic on-line, in place calibration, and failure detection and reporting.
2. Description of the Prior Art
The concentration of oxygen in processes containing water is important in many industrial processes and is critical for processes based on oxygen-using microorganisms; for example, fermentation, recombinant DNA processes, and waste water treatment. The present real time measurement of oxygen in industrial processes utilizes two well known forms of sensors: galvanic (voltage); and amperometric (current). The signal developed by the sensor is in response to the diffusion of gases across the membrane. Specifically, oxygen is reduced at the cathode in the sensor, generating either a potential or current. In either type sensor, the operation of the sensor is maintained such that the signal from the sensor is approximately linearly proportional to the partial pressure of oxygen or the concentration of dissolved oxygen.
There are a number of problems that can occur with this measurement system. For example, the membrane can become coated by the process or can change due to structural failure, such as cracks, leakage or degradation. Within the sensor body, the physical condition of the electrodes and the condition of the electrolyte directly affect the sensor signal current. Changes or degradation of these factors can occur in prior art systems without being detected, resulting in out-of-tolerance processes.
The most costly and error prone activity for any on-line, real time process measuring system is the repeated calibration of the system and sensor to maintain measurement accuracy.
Thus, there is a need for an oxygen measurement system in which defects in the oxygen sensor can be detected in real time on-line and accurate calibrations can be automatically performed on-line, thereby reducing operations cost and the probability of undetected failures which may produce catastrophic results.